Pathogenic Gram-negative bacteria cause a number of pathological conditions such as bacteraemia, bacteria-related diarrhoea, meningitis and (very commonly) urinary tract infections, i.a. pyelonephritis, cystitis, urethritis etc.
Urinary tract infections are one of the major causes of morbidity in females. Despite the overall importance of urinary tract infections in women, there have been few efforts to apply novel strategies in order to treat and/or prevent these diseases. Commonly, conventional antibiotics are used to treat these infections, such as treatment with penicillins, cephalosporins, aminoglycosides, sulfonamides and tetracyclines; in the special case of urinary tract infections, urinary antiseptics such as nitrofurantoin and nalidixic acid are employed, too. However, emerging antibiotic resistance will in the future hamper the ability to successfully treat urinary tract infections. Multiple antibiotic resistance among these uropathogens is increasing. It has been estimated that the annual cost for evaluation and treatment of women with urinary tract infections exceeds one billion dollars. In addition, approximately one-fourth of the yearly 4 billion dollar cost attributed to nosocomial infections is a consequence of urinary tract infections. Among the causative agents of urinary tract infections, Escherichia coli clearly predominates among Gram-negative bacteria.
Pathogenic gram negative bacteria, notably Escherichia coli, Haemophilus influenzae, Salmonella enteriditis, Salmonella typhimurium, Bordetella pertussis, Yersinia pestis, Yersinia enterocolitica, Helicobacter pylori, and Klebsiella pneumoniae owe part of their infectability to their ability to adhere to various epithelial tissues. Thus, e.g. E. coli adhere to the epithelial cells in the upper urinary tract in spite of the flushing effect of unidirectional flow of urine from the kidneys.
As indicated above, the above mentioned bacteria are involved in a variety of diseases: Urinary tract infections (E. coli), acute diarrhoea (E. coli, Y. enterocolitica and Salmonella spp), meningitis (E. coli and H. influenzae), whooping cough (B. pertussis), plague (Y. pestis), pneumonia and other respiratory tract infections (K. pneumoniae, H. influenzae) and peptic ulcer (H. pylori).
The initiation and persistence of many bacterial infections such as those described above is thought to require the presentation of adhesins on the surface of the microbe in accessible configurations which promote binding events that dictate whether extracellular colonization, internalization or other cellular responses will occur. Adhesins are often components of the long, thin, filamentous, heteropolymeric protein appendages known as pili, fimbriae, or fibrillae (these three terms will be used interchangeably herein). The bacterial attachment event is often the result of a stereo-chemical fit between an adhesin frequently located at the pilus tip and specific receptor architectures on host cells, often comprising carbohydrate structures in membrane associated glycoconjugates.
Uropathogenic strains of E. coli express P and type 1 pili that bind to receptors present in uroepithelial cells. The adhesin present at the tip of the P pilus, PapG (pilus associated polypeptide G), binds to the Galα(1-4)Gal moiety present in the globoseries of glycolipids, while the type 1 adhesin, FimH, binds D-mannose present in glycolipids and glycoproteins. Adhesive P pili are virulence determinants associated with pyelonephritic strains of E. coli whereas type 1 pili appear to be more common in E. coli causing cystitis. At least eleven genes are involved in the biosynthesis and expression of functional P pili; the DNA sequence of the entire pap gene cluster has been determined. P pili are composite heteropolymeric fibers consisting of flexible adhesive fibrillae joined end to end to pilus rods. The pilus rod is composed of repeating PapA protein subunits arranged in a right handed helical cylinder. Tip fibrillae which extend from the distal ends of each pilus rod were found to be composed mostly of repeating subunits of PapE arranged in an open helical conformation. The PapG adhesin was localized to the distal ends of the tip fibrillae, a location which is assumed to maximize its ability to recognize glycolipid receptors on eukaryotic cells. Two minor pilus components, PapF and PapK, are specialized adaptor proteins found in the tip fibrillum. PapF links the adhesin moiety to the fibrillum while PapK joins the fibrillum to the pilus rod. The composite architecture of the P pilus fiber reveals the strategy used by uropathogenic E. coli to present the PapG adhesin to eukaryotic receptors. The rigid PapA rod extends the adhesin away from interference caused by LPS and other components at the bacterial cell surface while the flexible fibrillum allows PapG steric freedom to recognize and bind to the digalactoside moiety on the uroepithelium. With a few exceptions, the structural organization of type 1 pili is very similar to that described for P pili. In type 1 pili, the mannose binding fibrillar tip adhesin is known as FimH.
The assembly of virulence-associated pili in Gram-negative pathogens requires the function of periplasmic chaperones. Molecular chaperones are vital components of all living cells, prokaryotic and eukaryotic. Chaperones serve a variety of cellular functions including folding, import and export of proteins in various cellular compartments (Gething and Sambrook, 1992). Thus, a periplasmic chaperone is a molecular chaperone which exerts its action in the periplasmic space in bacteria.
PapD and FimC are the periplasmic chaperones that mediate the assembly of P and type 1 pili, respectively. Detailed structural analyses have revealed that PapD is the prototype member of a conserved family of periplasmic chaperones in Gram-negative bacteria. These chaperones have a function which is part of a general strategy used by bacteria to cap and partition interactive subunits imported into the periplasmic space into assembly competent complexes, making non-productive interactions unfavourable. Determination of the three-dimensional structure of PapD revealed that it consists of two immunoglobulin-like domains oriented in a boomerang shape such that a cleft is formed. PapD binds to each of the pilus subunit types as they emerge from the cytoplasmic membrane and escorts them in assembly-competent, native-like conformations from the cytoplasmic membrane to outer membrane assembly sites comprised of PapC. PapC has been termed a molecular usher since it receives chaperone-subunit complexes and incorporates, or ushers, the subunits from the chaperone complex into the growing pilus in a defined order.
With the exception of the type IV class of pili, all other genetically well characterized pilus systems in Gram-negative prokaryotes contain a gene analogous to PapD (Normark et al., 1986; Hultgren et al. 1991); cf. also table A. FanE, faeE, sfaE, ClpE and f17-D have been sequenced (Lintermans, 1990; Schmoll et al., 1990; Bertin Y et al., 1993; Bakker et al., 1991) and encode pilus chaperones required for the assembly of K99, K88, S and F17 pili, respectively, in E. coli. The assembly of Klebsiella pneumoniae type 3 pili and Haemophilus influenzae type b pili requires the mrkb and hifB gene products, respectively (Gerlach et al., 1989,; Allen et al., 1991). The structure-function relationships of all of these chaperones have been analyzed using their amino acid sequences and information from the crystal structure of PapD (Anders Holmgren et al., 1992). The results have provided insight into the molecular intricacies that have been evolutionarily conserved in this class of proteins and suggested significant structural similarities to immunoglobulins.
PapD is thus the prototype member of a family of periplasmic chaperone proteins which are necessary for the correct supramolocular assembly of bacterial pili. Chaperones such as PapD in E. coli are required to bind the above-mentioned pilus proteins imported into the periplasmic space, partition them into assembly competent complexes and prevent non-productive aggregation of the subunits in the periplasm (Dodson et al., 1993).
In the absence of an interaction with the chaperone, pilus subunits aggregate and are proteolytically degraded (Kuehn, Normark, and Hultgren, 1991). It has recently been discovered (Strauch, Johnson and Beckwith, 1989) that the DegP protease is greatly responsible for the degradation of pilin subunits in the absence of the chaperone. This discovery has allowed the elucidation of the fate of pilus subunits expressed in the presence or absence of the chaperone using monospecific antisera in western blots of cytoplasmic membrane, outer membrane and periplasmic proteins prepared according to standard procedures. Expression of papG or papA in the degP41 strain (a DegP− E. coli strain) in the absence of a chaperone was toxic to the bacteria due to the accumulation of these proteins in the cytoplasmic membrane suggesting that the chaperone was required for subunit import into the periplasmic space. The severity of the growth defect was related to the level of expression of the pilin subunit, and was generally more dramatic with PapG than PapA. Co-expression of PapD under the control of the inducible arabinose promoter rescued the growth defect associated with subunit expression in the degP41 strain and allowed PapG to be imported into the periplasm.
Up to this time, little has been elucidated about the molecular recognition motifs or chaperones. Cytoplasmic chaperones such as SecB, GroEL and DNaK bind to a diverse group of unfolded target proteins in a sequence independent manner (Gething and Sambrook, 1992). Recently, it has been suggested that DNaK binds mainly via the target's peptide backbone (Landry et al., 1992) and that GroEL may rely more on side-chain hydrophobicity and the ability of the target sequence to form an amphipathic α-helix (Landiy and Gierasch, 1991). PapD differs, however, in that it seems to bind its target proteins in folded conformations (Kuehn et al., 1991).
The three-dimensional structure of PapD has previously been solved (Holmgren and Bränden, 1989). This has shown PapD to consist of two globular domains positioned such that the overall shape of the molecule resembles a boomerang with a cleft between the two domains. Each domain is a β-barrel structure formed by two antiparallel β-pleated sheets, packed tightly together to form a hydrophobic core, with a topology similar to that of an immunoglobulin fold. The N-terminal domain of PapD most resembles an Ig variable domain whilst the C-terminal domain of PapD resembles CD4 (Wang et al., 1990, Ryu et al., 1990) and the human growth hormone receptor (de Vos et al., 1992).
A structural alignment between PapD and several periplasmic chaperones predicted to have a similar immunoglobulin-like structure has identified invariant, highly conserved and variable residues within this protein family (Holmgren et al., 1992). Most conserved residues seem to participate in maintaining the overall structure and orientation of the domains towards one another. However, two conserved residues, Arg-8 and Lys-112 are surface exposed and oriented towards the cleft between the domains. Site-directed mutagenesis of the Arg-8 amino acid has shown that it form at least part of the pilus subunit binding pocket (Holmgren et al., 1992; Kuehn et al., 1993).
From a sequence analysis of a number of the above-mentioned pilus subunit proteins, it has been observed that they possess a number of common features including homologies at the C termini (see also example 2). It is thought that these similarities in sequence may be responsible for some function common to all the pilus proteins, such as binding to their periplasmic chaperone. Indeed, the C-terminal region of the P-pili adhesin PapG has already been shown to be important in the in vivo binding to PapD (Hultgren et al., 1989). Table A lists 16 periplasmic proteins, all involved in assembly of cell surface structures in pathogenic bacteria and all with significant homology with PapD.
TABLE AStructureOrganismChaperoneAssembledReferenceE. coliPapDP piliLund et al., 1987E. coli*FimCType 1 piliKlemm et al., 1992E. coliSfaeS piliSchmoll et al., 1990E. coliFaeEK88 piliBakker et al., 1991E. coliFanEK99 piliBakker et al., 1991E. coliCS3-1CS3 piliJalajakumari et al.,1989E. coliF17DF17 piliLintermans et al.,1990E. coliC1pECS31 ABertin et al., 1993E. coliEcpDnot identifiedRaina et al., 1993E. coliCssCAntigen CS6E. coliNfaENonfimbrial adhesin1E. coliAggDAggregative Adhe-rence Fimbria 1K. pneumoniaeMrkBType 3 piliAllen et al., 1991B. pertussisFimBType 2 & 3 piliLocht et al., 1992;Willems et al. 1992S. enteriditisSefBunknown piliClouthier et al.,1993S. typhimuriumPefDPEFH. influenzaeHifBunknown piliSmith et al., 1993Y. enterocoliticaMyfBMyf fibrillaeIriarte et al., 1993Y. pestisPsaBpH6 antigenLindler et al., 1992Y. pestisCaf1MF1 envelope antigenGalyov et al., 1991P. mirabilisMrpDMR/P Fimbriae?YehC?*This chaperone is present in all Enterobacteriaceae, since all members of this family produce type 1 pili. 
In summary, the three-dimensional structure of PapD as well as the function of PapD and other periplasmic chaperones are known, whereas the exact motif of binding between PapD and the pilus subunits has been unknown until now.